Abstract

Long-term geochemical monitoring performed in the seismic area of the Umbria-Marche region of Italy (i.e. Central
Apennines) has allowed us to create a model of the circulation of fluids and interpret the temporal chemical
and isotopic variations of both the thermal springs as well as the gas vents. Coincident with the last seismic crisis,
which struck the region in 1997-1998, an enhanced CO2 degassing on a regional scale caused a pH-drop in
all the thermal waters as a consequence of CO2 dissolution. Furthermore, much higher 3He/4He isotope ratios
pointed to a slight mantle-derived contribution. Radon activity increased to well above the ±2 sinterval of the earlier
seismic period, after which it abruptly decreased to very low levels a few days before the occurrence of the
single deep-located shock (March 26, 1998, 51 km deep). The anomalous CO2 discharge was closely related to
the extensional movement of the normal faults responsible for the Mw 5.7, 6.0 and 5.6 main shocks that characterized
the earlier seismic phase. In contrast, a clear compressive sign is recognizable in the transient disappearance
of the deep-originating components related to the Mw 5.3, 51 km-deep event that occurred on March 26,
1998. Anomalies were detected concomitantly with the seismicity, although they also occurred after the seismic
crisis had terminated. We argue that the observed geochemical anomalies were driven by rock permeability
changes induced by crustal deformations, and we describe how, in the absence of any release of elastic energy, the
detection of anomalies reveals that a seismogenic process is developing. Indeed, comprehensive, long-term geochemical
monitoring can provide new tools allowing us to better understand the development of seismogenesis.